To make a conductor easier to bend and for reducing eddy currents, a conductor of power transformer winding can include several parallel flat copper or aluminium strand conductors instead of one big rectangular one.
Each strand conductor has an insulation layer on its surface allowing the potential difference between adjacent strand conductors and prohibiting an eddy current from one strand conductor to another.
However, the respective lengths of parallel strand conductors can become different from one another as the average distance from the transformer core is different. As a result, the induced voltage between strand conductors becomes different from each other, and due to galvanic connections of both ends, circulating currents exist, which heats up the winding. Also, the load current does not divide equally to each strand conductor, which decreases the total load capacity. Two solutions have been proposed to avoid these problems.
A known winding structure has a mirror-image type transposing cross-over between each two adjacent disc, wherein each individual strand conductor on each cross-over is bent separately. However, a significant amount of separate strand conductors must be bent manually and carefully insulated according to this technique.
In FIG. 6 of WO03/067616, a disc winding is axially shared in a plurality of sections, and each cross-over in a section is a type where all strand conductors are bent essentially parallel. A twin-type transposing cross-over between each two adjacent sections brings the current and voltage balance between the strand conductors. The winding can be shared to as many axial sections as there are parallel copper or aluminium strand conductors in a conductor to achieve a good voltage balance. Alternatively, the winding could be shared to the number of sections, which number is a multiple of the amount of parallel strand conductors.
In a case of an even number of sections, such as six or eight sections, there are an odd number of transposing cross-overs between sections, and a mirror-image transposing cross-over is located in the middle of the winding making the cross-over location optimization calculation procedure needless.
In FIG. 7 of WO03/067616, a disc winding is close to the same as it was in FIG. 6, but the complicated mirror-image transposing cross-over at the middle of the winding has been replaced by a half and half type transposing cross-over.
A pneumatic, hydraulic or electrically driven hand tool, for example, can be implemented as a parallel bending means for bending each parallel strand conductors in one stage.
The above-mentioned progressive windings are fine for full turn discs, but cannot be used for windings with half turn discs. The use of half turns increases the flexibility of designing the windings to optimize the manufacturing process.
A lot of mechanical power is needed to bend several strand conductors parallel. Semiautomatic winding machines can be equipped with two bending heads for making twin-type cross-overs. For making a parallel bending, the strand conductors are to be shared between these two bending heads to share the mechanical power needed for bending. A strict positioning of the two bunches of the strand conductors for bending heads is needed to obtain a really parallel bending and to have it in one spacer span between two adjacent spacers. The insulation paper which is used for each second strand conductor broadens the parallel bended conductor. Accordingly, there is a desire to make the windings more suitable for semiautomatic winding machines in general and especially for half turn discs.